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Physics - Power and Momentum

When we talk about power, we’re talking about how much work gets done per second. 

Rule Number 14: 
Definition of power, power equals work divided by time or P=W/t. 

Power is measured in watts and 1 watt equals 1 joule per second. If we want to know how much force is necessary to keep a rocket in motion, we use the formula F=MA. Now, if we want to know how much power is necessary to keep the rocket in motion, we use this formula. 

Rule Number 15: 
Power necessary to keep a body in motion at constant velocity equals force necessary to keep it in motion times the velocity or P=Fv. Power equals work per second equals mass times acceleration times displacement divided by time. 

So, there are two formulas for power. When you want to know how much power is necessary to keep a body in motion at a constant velocity, you use this formula, P=Fv. When you want to know how much power is necessary to move a body through a particular space, you use P=W/t. 


Momentum is the product of the mass and velocity of an object. You should know that momentum is conserved just like energy. 

Rule Number 16: 
Definition of momentum, momentum equals mass times velocity or p=mv. This formula applies only for bodies traveling in a straight line. Therefore, it is often referred to as linear momentum. 

We know of course that for every reaction there’s an equal and opposite reaction. Closely related to that principle is this one, momentum is conserved. Think about two objects, a and b bumping into each other. Before colliding, each object has a velocity and a mass and hence, a momentum. The total momentum for the system, the two objects, is mv of object a plus mv of object b. 

Now, a collision between a and b will upset the situation. After the collision, each object may be traveling at a velocity different from the one with which it began. If that happens, each object will experience a change in mv which means each will experience a change in momentum. But total momentum for the two objects won’t change. So if the velocity of one object decreases, the velocity of the other will increase proportionately in order to maintain the same total momentum before and after the collision. 


A collision occurs when two or more objects strike one another. The important detail is whether or not they stick together. Inelastic collisions occur when things collide and stick. Elastic collisions occur when things bounce of one another. 

As you know, energy can neither be created nor destroyed. In a pure, elastic collision, the kinetic energy of the objects before and after the collision will be the same. However, this is not true of inelastic collisions. 

During inelastic collisions, some energy is lost to the collision itself. This is not to say that the energy has disappeared rather it has been changed into different forms like heat. 

Here is what you need to understand about collisions. Kinetic energy is conserved in elastic collisions. Kinetic energy is not conserved in inelastic collisions. Momentum is always conserved. 

You should know the first and second laws of thermodynamics. The first law of thermodynamics states that energy can’t be created or destroyed. Energy can change forms but it can’t disappear. Sometimes, the first law of thermodynamics is called the law of conservation of energy. In other words, the total amount of energy in the universe is conserved. It remains the same. 

The second law of thermodynamics states that the universe tends toward maximum randomness or entropy.